In a “hybrid automatic repeat request” (HARQ) protocol for a communication system, a part of data (which had been transmitted properly) from an earlier transmission of a communication message may be combined with another part of data which had not been transmitted properly previously from a later re-transmission of the communication message. HARQ may be considered as a variation of an ARQ (automatic repeat request) protocol error control method.
A given HARQ process in LTE (Long Term Evolution of 3rd Generation Partnership telecommunication systems) operates according to a stop-and-wait ARQ protocol. Each HARQ process corresponds to a subframe (or transmission time interval, TTI) where transmissions of that process may occur. Uplink transmissions and downlink feedback are associated to each other, for instance for sending ARQ responses, wherein corresponding transmissions are shifted by, e.g., four subframes with respect to the associated transmission in the opposite direction. In order to allow for continuous transmission the number of HARQ processes is equal to the number of subframes within one round-trip time (RTT).
In LTE FDD (frequency division duplex) all subframes of a radio frame are either UL (uplink) or DL (downlink) subframes. Transmission timing is defined so that the resulting RTT is 8 ms, hence 8 UL HARQ processes are needed for continuous transmission (3GPP TS 36.213, Physical Layer Procedures, V 9.1.0).
In LTE TDD (time division duplex) subframes of a radio frame are either UL or DL subframes depending on the TDD configuration. The transmission timing is defined so that the RTT depends on the TDD configuration and on the subframe number (see Table 8.1 of 3GPP TS 36.213, Physical Layer Procedures, V 9.1.0).
FIG. 1 shows a relay scenario. A scheduler SCenb in a radio access node eNB 20 schedules the data transmissions and allocates transmission resources to the relay nodes (RN) 10 (only one is shown in FIG. 1) and user equipments (UE) 50 in its coverage area. The eNB 20 may be in contact with some UEs 50 (only one is shown in FIG. 1) directly without a RN 10. This connection is indicated only schematically in FIG. 1. Another scheduler SCm is located in each relay node 10 and it allocates transmission resources to its associated UEs 30. Generally, a SCm may only allocate resources for Uu transmission, i.e. transmission between UE 30 and RN 10, that are not scheduled for Un transmission, i.e. transmission between eNB 20 and RN 10. An interface between the UE 50 and the eNB 20 or the UE 30 and the Relay Node 10 may be denoted as Uu interface. An interface between the relay node 10 and the donor eNodeB 20 may be denoted as a Un interface.
Referring to FIG. 1, the user equipment 30 (such as a mobile phone) can communicate in an uplink direction via a communication link 40 with the relay node 10. Furthermore, it is possible that the relay node 10 communicates with the user equipment 30 in a downlink connection via a communication link 42. Furthermore, the relay node 10 can communicate with the radio access node 20 in an uplink direction via a communication link 44. The radio access node 20 can communicate with the relay node 10 in a downlink direction via a communication link 46.
When supporting relays, relay node subframes are either allocated to the backhaul (eNB-RN or Un) or to the access link (RN-UE or Uu). The Un HARQ processes operate on the Un subframes and the Uu HARQ processes generally operate on the Uu subframes although DL control signaling for Uu HARQ processes may also occur in DL Un subframes, e.g. in a corresponding control region of MBSFN (Multicast Broadcast Single Frequency Network) subframes. If not all subframes are declared as Un subframe, less Un transmission opportunities occur within one HARQ RTT and consequently, less HARQ processes are required to fully utilize the link.
In LTE systems, DL Un subframes are MBSFN subframes, which are restricted in DL to subframes 1, 2, 3, 6, 7, 8 in FDD and 2, 3, 4, 7, 8, 9 in TDD. In DL, non-MBSFN subframes can thus not be Un subframes. The corresponding UL subframes cannot be Un either although in UL there are no MBSFN subframes. MBSFN subframes are either configured with a 10 ms period (one radio frame) using a 6-bit bitmap or with a 40-ms period (four radio frames) using a 24-bit bitmap (3GPP TS 36.331, Radio Resource Control (RRC), V 9.1.0). According to the configuration, subframes may be individually allocated by the scheduler.
Un subframes are configured by the eNB 20. Hence the number of Un subframes as well as the location and the period depends on the actual Un subframe configuration. The transmission timing and the number of Un UL HARQ processes cannot be pre-defined as today or would result in unsuitably large configuration tables.
If the same number of processes is configured that would be required without relaying according to the synchronous HARQ protocol used for the present LTE uplink, HARQ processes would not be served in every HARQ RTT. This would result in larger delays as retransmissions of a HARQ process can be performed only after all other HARQ process have been served, which according to the above explanation, takes longer than the HARQ RTT.
Correspondingly, ARQ technology may suffer from an inflexible or inefficient management of ARQ processes.